In a variety of applications, it is desirable to apply organic or inorganic particles, such as metal particles and metal powders, to substrates in a predetermined pattern. One such application is in the formation of patterns of solder metal or metal alloy particles to form solderable surfaces and structures on electronic parts and components.
The solder metal or metal alloy particles are suspended in a highly concentrated bath, or toner. Typical toners for electrokinetic deposition of tin powder, for example, consist of solvent (such as ISOPAR), charge director (such as polylecithin), and metal particles.
In electrokinetic or electrostatic deposition, the powder particles possess an electrical charge. To impart the charge, the surfaces of metal particles are pretreated by coating with an acidic polymer. Such a polymer will react with the charge director in the toner and create electrical charge on the surface of the particle necessary for electrokinetic or electrostatic deposition.
According to the conventional method of preparing the solder metal particles, the electrical charge is imparted to the powder, or to particles or spheres that are used, to make it electrokinetic by coating it with a charge control material. This involves, for example, coating the powder with a dielectric polymer material in solvent such as Joncryl 682 available from Johnson Polymer of Sturdevant, Wis. dissolved in IPA. Coating occurs in a fluidized bed coater. The purpose of the dielectric polymer is to provide a very acidic or hydroxyl surface with which the subsequently applied charge director can chemically react. The coated powder is subsequently wetted with a liquid having suitable dielectric properties, such as Isopar-G available from Exxon Mobil. Another suitable liquid is a product available from 3-M Corporation under the trade name Fluorinert. The powder constitutes about 0.5 to about 50 wt. % of the powder/liquid mixture, and about 1 to about 20 vol. % of the powder/liquid mixture. The powder is then dried and sieved to remove clumps and achieve a uniform powder size. The powder is then added to a charge control material such as poly lecithin electrolyte in an amount between about 10 and 20,000 microliters charge control material per liter Isopar.
Such a method has two distinct shortcomings: (1) the method of preparation is laborious and requires special equipment, and (2) polymer coated toner powders exhibit poor reflow characteristics. The method of preparation of the conventional toner requires coating on a fluidized bed coater, a specialized and expensive piece of equipment. The polymer coated toner must also be sieved because the toner powder clumps in conventional toners. This additional process step lowers the amount of usable product. Moreover, the polymer coating interferes with the reflow process which usually follows the deposition. Since the polymer is not completely soluble in the flux, the polymer leaves a solid residue on the reflow surface. This is problematic because the residues impede the electrical conductivity of the joint and can form a coating between the solder and the soldered electronic part, thus lessening the strength of the soldered joint.
Therefore, a need exists for a simpler method for preparing an electrokinetic or electrostatic deposition toner that eliminates the need for the special equipment and laborious process involved in the preparation of conventional toners. Further, there exists a need for a toner for imparting a charge on the surface of the tin particles that uses a film which does not interfere with the reflow process.